Ocean updates served in stragglers, flocks, and set waves

Archive for May, 2007

This just in: the road trip is still in progress and gradually getting sillier. In our whoosh across the Northwest, we’ve walked a weimeraner in the pouring rain, watched five-year-olds on a backyard zipline, stood in the sopping updraft of a Cascades waterfall, glissaded on the shoulders of Mt. Rainier, flown over Mt. St. Helens in a plane that might once have been a VW bug, explored a basalt cave, and found some amusing uses for lichen.

The Scribbler is helping a certain paleoceanographer drive across the country toward the esteemed paleoceanography department at University of California, Santa Cruz.

This special update comes to you from Bozeman, Montana. There’s a lot of geology out here, much of it kindly heaved into view, thousands of feet into the sky, by some ancient but obliging cataclysm. Every road cut, canyon wall, and scenic viewpoint is striped with the hardened remains of ancient sea floors. We’re reading John McPhee and trying to keep up.

So far we have endured an altitude headache (or the Scribbler did, after waking up at 13 feet elevation, flying to Denver, and driving to 9,600 feet). We camped under a bright Venus and the purring whistles of a boreal owl. We’ve spied soaring Swainson’s and ferruginous hawks, watched Old Faithful spurting dutifully at the clouds, and passed enormous statues of elk, grizzlies, bighorn sheep, pronghorn and jackalopes. We’ve crossed the Continental Divide six times so far.

This morning a storm swept through, the temperature dropped 20 degrees, and black rosy-finches were on the porch, eating sunflower seeds, flicking their pink wings.

They saw evidence of two distinct, massive burps of carbon dioxide, one lasting 3,000 years and beginning about 18,000 years ago; the other following on its heels about 14,000 years ago and, like its predecessor, lasting about as long as all of Western civilization so far. The scientists calculate that the CO2 came from water that had been submerged for at least 4,000 years – 1,300 years longer than the oldest water we know about in today’s ocean.

The nice part about this finding is that it plugs a gap in our knowledge. We’ve known for some time that atmospheric CO2 levels rose – and, curiously, radiocarbon levels fell – as the glaciers retreated. We just couldn’t be sure where it all came from.

But how does water get to be “old” anyway? That’s where the radiocarbon comes in. All of us have at least a hazy understanding that we can age things like Egyptian artifacts by comparing how much radiocarbon (C-14) they contain relative to regular carbon (C-12). The reason it works is that while something’s alive, its tissues pretty closely reflect the radiocarbon levels in the atmosphere. When the tissue dies, the C-14 begins a steady decay while the C-12 remains stable: so the ratio lets us back-calculate its age. This is why you can’t use carbon dating to find out how old something is, you can only find out how long it’s been dead.

Ocean water isn’t alive, but it does move around a lot, and it mixes surprisingly poorly. So when chunks of water sink below the surface they can wander the ocean depths for centuries, the water clinging to itself like a ghost wrapped up in its own shroud.

Now, radiocarbon is only made high up in the atmosphere, where cosmic rays bash into regular carbon atoms, making C-14 that rains down on us in a sort of high-energy game of bagatelle (oops – they bash into nitrogen atoms; see comment). What this means for water is that when it sinks below the ocean surface, it’s like a dead Egyptian artifact, cut off from its source of radiocarbon. The water starts recording its age immediately.**

So putting it all together, Marchitto found evidence – in the shells of 18,000 year old protists – of 4,000 year old deep water moving around in the upper ocean. (To stretch an analogy, it’s as if the ghosts in the cellar had gotten restless and moved up to the ground floor). He and his colleagues think much of that water reached the surface and came back into contact with the atmosphere.

Like the burps of a Scribbler drinking a tamarindo-flavored Jarritos, only considerably larger, these would have raised the carbon dioxide level in the atmosphere. But because the water had been submerged so long, the burps would have been much less radioactive than the Scribbler’s (who contains only the most up-to-date radiocarbon). And because we’re talking about so much carbon dioxide, the overall effect would be an observable dip in the radiocarbon signature of the atmosphere – one that’s been puzzled over for some time in the Greenland ice cores.

In “Deglaciation Mysteries,” Ralph Keeling, of Scripps Institution of Oceanography, offers his perspective on the research, in the same issue of Science. For readers who want something more technical than this post, but less technical than the full paper.)

**This neat trick is one of the main ways oceanographers map the deep currents of the ocean, and it’s how we know that once carbon makes it below the surface waters, it can drop out of the climate picture for millennia.

Radiocarbon is phenomenally useful in other situations, too: It helps us detect manmade organic pollutants when we find them, because they’re made from petroleum, and petroleum is very, very old (so its radiocarbon ratio drops off the chart). And if you’ve ever heard someone say that atmospheric CO2 comes from forest fires rather than fossil fuel emissions? Radiocarbon lets us put a number on that claim.

A paper last week in Science reached back 38,000 years to trace how the ocean dumped heaps of carbon dioxide into the atmosphere just as the last glaciation was starting its decline. Tom Marchitto and colleagues discovered that around 18,000 years ago, atmospheric carbon dioxide began its steady rise from 180 ppm to the oft-quoted 280 ppm before the start of the industrial revolution. They think the CO2 came from very old, very deep ocean water that burst to the surface in two prolonged belches.

You could be forgiven for wondering how we’re so sure what the molecular composition of air and ocean water were 14,000 years before the pyramids had been built. Paleoceanographic research is a scavenger hunt of bizarre techniques on unlikely objects: sea mud; old ice; corals.

First you bring up some seafloor mud in what is essentially a very long soda straw. Put it under a microscope and pull out the shells of tiny dead creatures called forams (Not plants, not animals; they’re protists.). During their brief but happy lives, some of these floated in the surface water while others lived on the seafloor. Learn how to tell them apart, and you can compare their radiocarbon ages – along with oxygen isotopes – to surmise how the deep water was different from the surface water way back then.

If that sounds shaky, there are at least supplementary techniques that scientists use to make sure they’re in the right ballpark. Some 3-km deep holes in the Greenland ice sheet (and Antarctica) provide similar information from gases caught in the annual snow layers. People actually count, layer by layer, 100,000 years into the past. When they see a series of spikes in ice-core isotopes mirrored in seafloor mud isotopes, they can be reasonably sure they’re looking at the same time in prehistory. In the pages of Science, these are called “tie points.” In the bar after work, it’s called “wiggle matching.”

Other people pull up coral from the seafloor and look at heavy elements trapped in its layers. A neat trick with the way uranium transmogrifies into thorium and protactinium as it decays – and how those elements tend to sink differently – lets them figure out the volume of ocean currents in the past.

With me so far? Me neither. This is one reason why it’s so hard to find good articles about paleoceanography in People or Reader’s Digest: too much background.

Tomorrow: We’ll step back and just think about radiocarbon. Everybody knows what carbon dating is. But how does it work? And what does it tell us about the ocean?

Cheers to New Scientist for their set of interrelated stories addressing myths and misconceptions about climate change science. They start by acknowledging how hard it is to keep straight the complex actions, interactions and feedbacks that shape Earth’s climate, even without some “other side of the debate” throwing up roadblocks.

Contorted evidence and factoids can at times arrive in flurries, making them difficult to refute: the sun’s output is changing; cosmic rays are to blame; we can solve it by fertilizing the ocean; etc.

The pieces are short, but that’s a bonus. If you’ve got time on your hands, you can read exquisitely detailed discussions, buttressed by long-term data sets, on any one of these topics at places like RealClimate.org. But those discussions run to the thousands of words – and dozens of graph traces – before you even get to the comments section.

What New Scientist has done is distill what’s wrong with each of these 26 common misconceptions, then collect them all in a single place. Put it in your back pocket this summer, for when you head off to neighborhood barbecues and the like.

Way back when the Scribbler was a feisty young field biologist staking out nests of strange Neotropical antpittas, I often chewed grass stems to keep myself awake. Occasionally, I would panic to find that a seed head was working itself stubbornly down my throat, pointy end first. When I tried to spit it out, little hairs would anchor it to my cheek, and every time I swallowed it would slide farther back toward the soft and presumably important parts of my throat. If you’ve never tried this, don’t: Choking to death on a hayseed is both frightening and kind of pathetic at the same time.

If only I had realized there was a Science paper in there somewhere, I might not have felt so humiliated.

From a mechanistic point of view, we have discovered a device for movement that is composed of passive elements.

What they found was that the wheat awns – those are the long, stiff hairs that poke up off the wheat seed – are asymmetrical in cross section. When they’re moist, the awns are more or less straight, but when they’re dry one side of the awn shortens more than the other, causing the awn to curl.

Combine that with the natural cycle of humidity in a day and the action of those little side-hairs that got stuck in my throat, and the awns can actually propel the seeds over the ground or down into the soil. No, really.

As the day heats up and the seed dries, the awns bend and the stiff little side hairs grip the soil. When night falls, the cool air moistens and lengthens the awns. They straighten, but the hairs allow that straightening to happen only in one direction, pushing the pointy seed forward or down.

Voila: Wiggles in the humidity record induce wiggles in the wheat awns (which the authors note, in a nice touch, “resembles the swimming stroke of frog legs”), and the seed buries itself. Next day: more wiggles, more progress for the wheat seed.

(No measurements yet on the force with which the point is driven into the ground. Or whether the power involved could be, uh, harvested, to help those poor underpowered krill mix the oceans.)

Longtime readers may remember a cool story about very small ocean creatures mixing the water column with their daily, en masse commutes. A Canadian study had calculated the amount of power exerted by all those millions of tiny, simultaneous wiggles and it was roughly equal to the amount supplied by winds or tides. This was big news – a huge source of power for mixing the ocean,*** hiding in the tiny pink muscles of krill.

Unfortunately, André Visser has gone on record in this week’s Science to puncture that bubble of enthusiasm. Give him credit for letting us down easy, though. His endearing first sentence:

“Every now and then, an idea comes along that is so appealing, it seems bad manners to challenge it.”

Visser points out, using only three Greek letters and two fractional exponents, that although the power all those krill produce is tremendous, it doesn’t mean much mixing gets done. That’s because to mix water you don’t just impart energy on it. You need to get it to form eddies or currents that move far enough to run into some different water. For a krill that’s 1.5 centimeters long, it’s hard to push water that far, no matter how hard they wiggle.

A krill’s “mixing efficiency,” Visser estimates, hovers between 0.01 percent and 1 percent of the mechanical energy of its wiggling. That is to say, between 99 percent and 99.99 percent of the krill’s efforts vanish almost immediately into heat. No word yet as to how good krill are at heating the ocean.

***So who cares? Well, it turns out that mixing the ocean is a lot harder than it sounds. Think about adding half-and-half to your coffee without being allowed to stir it. At first, in a clear mug, it would look pretty cool. But as the cream stalled out in mini-eddies or pooled at the bottom of the mug you would quickly get impatient. Especially if your mug covered 2/3 of the Earth’s surface and was 4,000 meters deep on average. And mixing turns out to be crucial for all sorts of planetary chores: moving heat around, absorbing or releasing carbon dioxide, and stirring nutrients from where they’ve fallen on the sea floor up to the sunlight, where plankton can use them.

About the Scribbler

Hugh Powell is a little weary of big-ticket items like Pluto, the Mars rover, and small fossilized humans getting all the science news coverage. Keep an eye out here for wisps and scraps you won't find anywhere else. Particularly about the ocean, which is really cool and, honestly speaking, much bigger than you think.